The present invention relates to a sensor module for a probe head of a tactile coordinate measuring machine, and to a probe head comprising such a sensor module and a stylus attached to the sensor module.
In the prior art, coordinate measuring machines are used inter alia for measuring the shape of a measurement object with high accuracy. For example, the shape of workpieces produced by machine may be checked in this way in the course of quality control. For the measurement process, the probe head of the coordinate measuring machine is moved towards the measurement object by means of a suitable movement mechanism until the stylus touches a desired measurement point on the measurement object. The spatial coordinates of the measurement point can then be determined from the position of the probe head and, if appropriate, from the relative position of the stylus with respect to the sensor module.
German patent application DE 101 08 774 A1 discloses a probe head in which the stylus is arranged on a lateral mount. In an embodiment, the lateral mount comprises a stationary frame having a square base area, at whose center a part is arranged which can move relative to the frame. The moveable part is sometimes referred to as a “boss” by the skilled persons, and it carries the stylus. In the described embodiment, the boss is connected to the frame either via four or via eight webs. When the stylus is deflected, the webs are twisted, and this can be evaluated by means of strain sensors arranged on the sensor module. In this embodiment, the frame, the webs and the boss are produced from a solid silicon body by an etching process.
The basic concept of such a sensor module is also discussed in an article by Kleine-Besten et al. entitled “Miniaturisierter 3D-Tastsensor für die Metrologie an Mikrostrukturen” [Miniaturized 3D Probe Sensor for Metrology of Microstructures], which appeared in the German Journal “tm—Technisches Messen” [tm—Technical Measurement], Issue 12/99, pages 490-495. This article describes investigation results on a semiconductor sensor module, wherein, in contrast to DE 101 08 774 A1, the boss of the sensor module is held on the frame via a single solid membrane. The use of individual webs for holding the boss, as disclosed by DE 101 08 774 A1, is mentioned in a brief outlook at the end of the article in connection with ideas for compensating different bending stiffnesses of the module in the three spatial directions. This is because the investigation of the sensor module having the solid membrane has shown that the bending stiffness is considerably less when the stylus is deflected in a plane parallel to the frame (X direction or Y direction) than when it is deflected perpendicularly to the frame (Z direction).
The use of webs for mounting the boss and the stylus rather than a solid membrane leads to some degree of matching the bending stiffnesses in the three special directions. However, there are still considerable difficulties in scanning measurement processes, i.e. measurement processes in which the probe head is guided in continuous contact with the measurement object (what is referred to as “scanning”). It is still very problematic to determine the deflection in the Z direction in such measurement scenarios, despite the matching of the bending stiffnesses already achieved so far by means of the webs.